It is known for optical components to be integrated into a planar optical circuit in monolithic or hybrid fashion. Examples of the components are optical phased arrays (AWG—arrayed waveguide grating), variable attenuator units (VOA—variable optical attenuator) and power monitoring devices (PM—power monitor) which detect the optical power in an optical waveguide of the planar optical circuit. Optical phased arrays are used in particular as wavelength division multiplexers and demultiplexers in WDM (wavelength division multiplex) and DWDM (dense wavelength division multiplex) based transmission links. Variable attenuator units make it possible to perform a channel-dependent attenuation of the levels of individual data channels of an array. Different levels of the optical channels can be equalized by means of the channel-dependent attenuation. Power monitoring is realized by means of photodiodes and serves for monitoring the signal powers in individual waveguides.
An ever present problem in planar optical circuits is the existence of undesirable scattered light. Scattered light arises for example in variable attenuator units, as is explained below with reference to FIGS. 9 and 10. An attenuator unit is realized for example by a Mach-Zehnder interferometer 100. A Mach-Zehnder interferometer 100 has an input waveguide 101 having an input power Pin, an input port 102, a first arm 103, a second arm 104, an output port 105 and an output waveguide 106 having the output power Pout. A heating element 110 is situated on one arm 103 of the Mach-Zehnder interferometer 100. Heating of the heating element changes the temperature in the corresponding arm 103 and the refractive index thereof. This results in a phase difference between the signals of the two arms 103, 104, which leads to a change in the output power Pout.
Provided that there is no phase difference between the two arms 103, 104, the output power Pout reaches a maximum, while the output power Pout is at a minimum given a phase difference of π. As soon as a phase difference occurs between the two arms 103, 104, a part of the light is in this case emitted or radiated from the optical waveguide. The emission of light is at a maximum given a phase difference of π.
The situation is then such that the emitted light is hardly absorbed by the light-guiding substrate of the planar optical circuit. Rather, the majority of the scattered light propagates arbitrarily in the substrate. FIG. 10 shows the simulation result for the field distribution in a Mach-Zehnder interferometer in accordance with FIG. 9, the phase difference between the two arms 103, 104 being π. The scattered light X propagates essentially conically proceeding from the output port 105, that is to say the confluence region of the two waveguide arms 103, 104. In this case, the intensity of the scattered light X is greatest in the vicinity of the output waveguide 106 and decreases with increasing distance from the output waveguide 106.
The scattered light X illustrated diagrammatically in FIG. 10 poses a problem from a number of standpoints. Firstly, it disturbs the function of photodiodes which are mounted in trenches or cutouts of the substrate and, by way of example, perform power monitoring of the optical signals of individual waveguides. The disturbance signal may reach the level of the useful signal in this case. A further problem is that the emitted scattered light may couple into adjacent channels and thus generates an undesired crosstalk.